Nobleite
Chemical formula: CaB<sub>6</sub>O<sub>9</sub>(OH)<sub>2</sub>·3H<sub>2</sub>O
Nobleite is a rare, hydrated calcium borate, forming colorless, prismatic crystals with a vitreous luster.
Properties
- Mohs hardness
- 3
- Luster
- Vitreous, Pearly
- Streak
- White
- Density
- 2.09
- Cleavage
- Perfect on {010}
- Fracture
- Uneven
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Nobleite can be identified by its characteristic, elongated, prismatic crystals, often forming radial aggregates. Key features include a vitreous to pearly luster, low hardness (about 3 on the Mohs scale), and perfect cleavage in one direction. It reacts with acids and is partially soluble in water. ## Distinguishing from Similar Minerals Nobleite is sometimes confused with other borates, such as Colemanite or Ulexite, which occur in similar environments. It differs from Colemanite by its lower hardness and different crystal habit. Ulexite, in turn, forms characteristic fibrous aggregates (the so-called "television stone"), while nobleite occurs as distinct prismatic crystals. ## Crystal Forms Nobleite crystals are monoclinic, prismatic in habit, elongated along one axis. They often occur as radial or tangled aggregates, as well as veins cutting through other borate minerals or clays.
Geological environment
## Genesis Nobleite is a secondary mineral, formed by the alteration of other borate minerals, such as Colemanite and Ulexite. It forms in evaporated salt lake (playa) deposits in arid climates. It originates under low temperature and pressure conditions, often in bentonitic clays. ## Mineral Associations This mineral co-occurs with other borates, such as Colemanite, Ulexite, Gowerite, as well as Gypsum and Calcite. ## Localities The most important and classic localities for nobleite are in Death Valley, Inyo County, California, USA, particularly in the Furnace Creek area (Gower Gulch and Boraxo mines). It is also known from the Bigadiç region in Turkey and from Salar de Uyuni in Bolivia.
Rarity
Rare
For collectors
## Quality Criteria The most prized nobleite specimens are those with well-formed, transparent, prismatic crystals, forming aesthetic, radial aggregates. The size of the crystals is also important – specimens with crystals exceeding 1 cm are already considered very good. Contrast with the matrix (e.g., dark clay) also enhances the specimen's attractiveness. ## Popular Localities Specimens from the type locality, Death Valley in California, are by far the most sought after by collectors. They are characterized by their classic form and often good crystal quality.
Care and storage
## Cleaning Nobleite specimens should be cleaned very carefully, using a soft brush to remove dust. Due to its partial solubility in water, contact with water should be avoided. If a liquid is necessary, alcohol is recommended, applied in small amounts and evaporating quickly. ## What to Avoid Contact with water, acids, and other chemicals should be strictly avoided. The mineral is sensitive to changes in humidity and temperature. It should not be heated or exposed to direct sunlight, which can lead to dehydration and damage to the crystals. ## Storage It is recommended to store nobleite in a dry place, preferably in a closed, airtight container or display case, to protect it from moisture and dust. Due to its softness and perfect cleavage, it should be stored separately from harder minerals to avoid scratches and mechanical damage.